1. Field of the Invention
The present invention relates to products (including machines, manufactures, and compositions of matters) and processes (including simple methods and production methods). In particular, one embodiment of the present invention relates to semiconductor devices, methods for driving the semiconductor devices, or methods for manufacturing the semiconductor devices. Further, one embodiment of the present invention particularly relates to processors as semiconductor devices and methods for driving the processors.
Note that in this specification, a semiconductor device means a circuit having a semiconductor element (e.g., a transistor or a diode) and a device having the circuit. The semiconductor device also means any device that can function by utilizing semiconductor characteristics. For example, an integrated circuit, a chip having an integrated circuit, a display device, a light-emitting device, a lighting device, and an electronic device are all semiconductor devices. Further, here, a processor means a processing unit having an arithmetic unit and a controller. For example, a microprocessor, a CPU, and an MPU are processors.
2. Description of the Related Art
These days, as a technology for downsizing a semiconductor element is improved, the integration degree of a processor such as a CPU or an MPU is markedly improved, and the processing speed of the processor is greatly increased. On the other hand, as the semiconductor element becomes small, the number of semiconductor elements provided in the processor greatly increases, and the power consumption of the semiconductor element due to leakage current markedly increases.
One of driving methods for reducing the power consumption of a processor is a method for driving a processor that is called power gating (hereinafter also referred to as a power gating driving method). A power gating driving method is a driving method in which in a period during which a processor does not need to perform arithmetic processing, supply of power to part of the processor (e.g., a logic unit) is stopped to prevent consumption of wasted power.
A volatile storage element such as a register or an SRAM is usually used as a storage element in a processor. When supply of power to a logic unit or the like in the processor is stopped by the power gating driving method, data in the volatile storage element provided in the logic unit is lost. In contrast, by providing a flash memory that is a nonvolatile storage element in a processor and writing data in a volatile storage element to the nonvolatile storage element when supply of power is stopped, the data can be retained (for example, see Patent Document 1).